The present invention relates in general to detecting misfires occurring during normal in-use vehicle operation of internal combustion engines, and more specifically to determining a baseline acceleration value in a manner that permits detection of high misfire rates, such as 50% misfires.
Monitoring of crankshaft acceleration in an internal combustion engine is a preferred technique to detect misfires of individual cylinder firings during engine operation. An advantageous system for crankshaft based misfire detection is described in U.S. Pat. Nos. 5,044,195; 5,056,360; and 5,109,695. This system utilizes the determination of a deviant acceleration, which is the difference between the instantaneous acceleration for an individual cylinder firing and an estimated baseline acceleration. In particular, the baseline acceleration is determined as a median average over a series of cylinder accelerations including the cylinder firing of interest.
The foregoing system achieves acceptable sensitivity at low misfire rates (i.e., provides reliable detection and a low false alarm rate) thereby making crankshaft-based misfire detection feasible. However, the typical method employed for determining a baseline acceleration lacks the ability to detect high misfire rates (specifically 50% of cylinder events being misfires). More specifically, prior averaging methods (including median averaging) are distorted unacceptably by the presence of data from misfires when those misfires comprise 50% of the data within the averaging window.
Although a 50% misfire rate may be a rare occurrence and although such a condition may be immediately apparent from roughness in engine operation, it may nevertheless be desirable to electronically detect such a condition so that it can be recorded and/or corrective action can be taken. Furthermore, for some engines and some types of ignition systems, a 50% misfire rate can result from a single component failure (e.g., a coil failure in a four-cylinder distributorless wasted-spark ignition system using two coils).